There is a thread from last year back a page or two on this Touring Forum comparing the 520 to the BG BLT. There are interesting comments, including (or some might say in addition to) my own.
I was weighing the 520 vs. the BLT myself last year. Without rehearsing my comments on the other thread, I can say that I bought the 520 because my LBS was reasonably generous in its sales offer. They replaced the 105 set with LX for free, and charged me only a small amount to replace the stock 520 rims with Mavic Touring rims. That left only two important differences between the BLT and the 520. First, the possibility that the BLT's frame walls were a bit thicker than the 520's. (I say possibility because Trek doesn't say for sure just how thick its frame walls are.) Second, the BLT would have been somewhere around $300 dollars (U.S.) more expensive.
The modern 520 frame has stood up to long tours all over the world, so I didn't see sufficient importance to the (possible) frame upgrade to buy the BLT.
So, in short, the decision for me was determined by the extent to which the LBS was willing to upgrade the stock 520 cheaply.
As far as the Giant vs. the 520, I wouldn't worry about the materials per se in the respective frames as much as the overall quality of frame builds from each company. Aluminum frame failures are rare in any bikes nowadays, including mtbs. However, the Giant Touring uses the same frame as the rest of the OCR racing line! That would give rise to a healthy suspicion in me. When I consider that performing the Giant experiment would cost me extra money too, it is hard to get too excited about this option. Yes, the AL frame does provide a significant weight reduction. And, yes, you get the disk brakes. But a road frame? I wouldn't do it.